This invention relates to systems for tracking targets with electromagnetic energy sensors, and more particularly to radar angle, range and velocity tracking systems.
Although reference will be made hereinafter to only active radar tracking systems, it is to be understood that the techniques of the present invention are also applicable to other electromagnetic systems, such as laser tracking systems, and to passive tracking systems as well. For example, to measure satellite orbits using a completely passive tracking system, either optical or UHF sensors measure angles and continually drive sensor positioning servomotors to maintain the sensor axis on the satellite. However, for determination of orbits without round-trip time-delay (range) measurements, long observation periods are required to determine the orbit. Consequently, active systems are preferred in satellite tracking as in most other applications but there will be those special applications where range information is not required or can be determined by some other means.
The concept of pulse trasmission has been basic to most tracking radar systems because range can be easily measured by the time delay of each pulse round trip. Information as to the speed of the target is determined by measuring the doppler frequency shift between transmitted and reflected waves of each pulse, and information as to the angular coordinates of the target is determined by relating received energy properties to the sensitivity and directivity patterns of the sensor (antenna).
In a typical radar tracking system, a target reflects pulses to an antenna which derives signals that provide the desired information. A trnsmitter and receiver is operated in accordance with a predetermined mode in regard to frequency and PRF, both of which are often selected automatically or by an operator for a particular target to avoid velocity and range blind zones. The receiver provides video signals to a radar signal processor which produces signals that are measurements of range, range rate (radial velocity) and angular coordinates (elevation and azimuth deflection) of the target. These measurement signals are then applied to standard antenna controllers for positioning the antenna and to range and velocity controllers. A typical radar tracking is thus a multiple closed-loop feedback system. Two loops for tracking in azimuth and elevation, and two loops for tracking in range and velocity. Standard servomechanism design procedures are both applicable and practical to each of these loops, but optimal results are not always achieved by such procedures.
The prior art tracking loops have ranged from classical single control loops, employing low-pass filtering to remove noise from measurement signals, to more advanced control loops employing filters for estimating quantities from measured differences between estimates and actual measurements, such as .DELTA.R.sub.m = R - R.sub.m. The timing of a range gate, R.sub.G, represents the estimate, R, and the time of arrival of a round-trip radar pulse represents the measured range R.sub.m. The difference then is .DELTA.R.sub.m. The range servo-loop continually updates the range estimate R to drive .DELTA.R.sub.m toward zero. Analogous servomechanism procedures drive the radial velocity difference, .DELTA.V.sub.m, toward zero and the azimuth and elevation angle differences, .DELTA..eta..sub.m and .DELTA..epsilon..sub.m, toward zero.
A typical tracking system in a tactical aircraft performs two principal functions. First, it positions the antenna and the range and velocity gates of the radar signal processor, so as to keep the target accurately centered within the "field of view" of those elements. Second, it generates estimates of the target's position and motion for use in fire-control and aircraft-steering computations. These functions must be accurately performed over an extremely broad range of operating conditions, ranging from highly dynamic dog-fight encounters at short-range to very low signal-to-noise ratios at long range.
Several factors must be dealt with in the design of the tracking system in order to obtain the required accuracies and performance capabilities. First, there is the problem of measurement errors due to factors such as noise, target scintillation, and radome refraction. Maneuvers of the target and deception techniques (countermeasures) employed by targets to prevent tracking are the second class of problems which must be dealt with.
Due to the large variations in the characteristics and magnitudes of these error sources over the wide spectrum of operating conditions, variable parameter filters are necessary to meet tracking accuracy requirements of high performance aircraft. For example, narrow bandwidth filtering is required for tracking targets at long range with low signal-to-noise ratios. But rapid response is necessary for tracking highly maneuverable targets at short range. These factors, plus the requirement for generating high accuracy estimates of target line-of-signt (LOS) rates, and the desirability of employing one unified set of general filter equations to handle any radar mode -- high PRF, medium PRF, low PRF, and so forth -- have led to the use of a new approach in the design of the tracking filter using Kalman filtering.
An object of this invention is to use the Kalman filter concept to provide a practical and readily-mechanizable approach to the optimal estimation of dynamically-varying quantities from noisy measurements (optimal in the sense of minimizing mean-square estimation errors) for a wide class of tracking problems.
The manner in which discriminants are processed for use in performing angle, range, and velocity tracking must depent upon the characteristics and quality of the discriminants. For example, at high signal-to-noise ratios, greater weighting can be applied to the discriminants than at low signal-to-noise ratios. And since signal characteristics can fluctuate rapidly over extremely wide ranges, some method of adapting the tracking-loops to these changes must be used in order to prevent severe degradations in tracking performance.
A further object of this invention is to provide means for automatically adapting tracking loops to rapid changes in signal characteristics so that optimal tracking can be maintained despite large variations and fluctuations in these characteristics.